Psychopath Test - Part I

Got this in the email, found it rather interesting (and yes, I failed miserably, could come up with no answer whatsoever, so I guess I'm not a psychopath):

[The test originally was all in one email and said to scroll down to get the answer, but I changed it to give you the answer tomorrow. Wouldn't want to spoil the surprise, and that much blank space is just a waste of blog-o-sphere.]

Read the question, come up with an answer, and tomorrow I will give you the answer. This is not a trick question. It is as it reads.

A woman, while at the funeral of her own mother, met a man who she did not know. She thought he was "amazing." She believed him to be her dream partner so much, that she fell in love with him right there, but never asked for his number and could not find him. A few days later she killed her sister.

Question: What was her motive for killing her sister?